Vinyl acetal polymers as represented by a vinyl butyral polymer have been used in various applications such as an intermediate film for a laminated glass and inks, paints, enamels for baking, wash primers, lacquers, dispersants, adhesives, ceramic green sheets, heat developing photosensitive materials, binders for a water-based ink demand layer and the like because such polymers are excellent in toughness, film forming properties, dispersion of an inorganic or organic powder such as a pigment in the polymer, adhesiveness to a coated surface and the like. The major reason for the use of a vinyl acetal polymer in various applications is that hydroxyl groups in the vinyl acetal polymer form hydrogen bondings, resulting in toughness of the polymer. On the other hand, such hydrogen bondings lead to somewhat poor flexibility.
Furthermore, a vinyl butyral polymer has a high glass transition temperature and less fluid even at a high temperature. Therefore, for thermoforming, an external plasticizer such as a phthalate of an alcohol having 6 to 10 carbon atoms, triethyleneglycol di-n-heptanoate, tri(2-ethylhexyl) phosphate and tricresyl phosphate is added to the polymer. If the plasticizer is not properly selected, phase separation may occur between the vinyl butyral polymer and the plasticizer, or contact of a composition containing the vinyl butyral polymer and the plasticizer with a solvent may cause elution of the composition, leading to significant variation in composition properties. Specifically, film strength, transparency and storage stability of a film or sheet produced by processing a vinyl butyral polymer may be deteriorated, or an ink may bleed on the surface of the film or the sheet after printing. They have been attempts for an internal plasticized vinyl acetal polymer for preventing deterioration in physical properties due to addition of a large amount of a plasticizer as described above and deterioration of physical properties due to bleeding or migration to another resin of the plasticizer.
Patent Reference No. 1 has described a ceramic slurry for molding a green sheet comprising a polyvinyl acetal resin prepared by acetalizing a modified polyvinyl alcohol having monomer unit containing a polyoxyalkylene (hereinafter, sometimes abbreviated as “POA”) group with an aldehyde, a ceramic powder, a plasticizer and an organic solvent. The polyvinyl acetal resin is believed to satisfactorily disperse a ceramic powder because the resin is contained as a solution in an organic solvent. It is also described that POA groups in a side chains in the polyvinyl acetal resin have internal plasticization effect, and that a green sheet produced using the polyvinyl acetal resin exhibits improved flexibility. However, the POA groups described in Patent Reference No. 1 are comprised of a single POA unit mainly including a polyoxyethylene group or a polyoxypropylene group. There is a problem that when a polyvinyl acetal resin having such a polyoxyalkylene group is processed into a film or a sheet, a resulting film or sheet becomes cloudy. Furthermore, a green sheet produced using a polyvinyl acetal resin having such POA groups has insufficient flexibility and unsatisfactorily disperses a ceramic powder.
Patent Reference No. 2 has described a vinyl acetal polymer produced by acetalizing a vinyl alcohol polymer (hereinafter, sometimes abbreviated as “PVA”) with a terminally etherized oxyethylene aldehydes/oxyalkanals. It is known that the polymer has features that it contains vinyl alcohol units in a high content, can be used for thermoforming without adding an external plasticizer and is highly adhesive to a metal and a glass. There is, however, a problem that a film or sheet produced by processing the vinyl acetal polymer thus produced becomes cloudy due to the remaining unreacted oxyethylene aldehydes/oxyalkanals in the vinyl acetal polymer.
To date, there have not been provided vinyl acetal polymers which exhibit internal plasticity while being highly tough and avoid phase separation when a resin composition is prepared from the polymer.